Historically, maize (corn) is an important crop for food, feed, and industrial uses. Any environmental stress factor, e.g. disease, that affects maize can have an impact on maize grain availability for these uses.
Gray Leaf Spot (hereinafter referred to as GLS) has gained prominence the last three decades and is a significant foliar disease in the United States and in other major corn producing areas, such as Mexico, Brazil, Europe, and South Africa. The incidence and severity of GLS appears to be increasing in the United States (Wang et al., Phytopathology 88:1269-75 (1998)), perhaps due to an increase of maize on maize plantings and reduced tillage. These conditions can contribute to overwintering of the fungus and early infection the following season (Laterall and Rossi, Plant Dis. 67:842-37 (1983)). Yield losses in excess of 50% have been reported during GLS epidemics in the United States (Laterall and Rossi, supra; Lipps, Plant Dis. 71:281 (1987)), and estimated losses have been as high as 100% where severe epidemics contributed to increased stalk lodging and early senescence (Laterall and Rossi, supra).
The fungal pathogen Cercospora zeae-maydis, which causes GLS, characteristically produces long, rectangular, grayish-tan leaf lesions which run parallel to the leaf veins (Tehon and Daniels, Mycologia 17:240-49 (1925); Latterell and Rossi, supra; Ward et al., Plant Dis. 83:884-95 (1999)). The lesions may blight part or all of the leaf and typically appear in the lower leaves first. Blighting due to GLS is associated with the premature loss of photosynthetic area. The dominant sink of the post-flowering maize plant is the ear, and blighting induces the plant to transfer photosynthate from the stalk and roots to the ear, at high levels, thus causing premature senescence and reduced yield.
The fast and effective development of maize varieties with GLS tolerance is beneficial. The level of tolerance to GLS in commercial hybrids and inbreds differs among varieties. Some varieties exhibiting strong tolerance have been reported. However, the use of phenotypic selection to introgress the GLS trait from a tolerant variety into a susceptible variety can be time consuming and difficult. GLS is sensitive to environmental conditions and requires high humidity and extended leaf wetness. This sensitivity makes it difficult to reliably select for GLS tolerance from year to year based solely on phenotype (Lehmensiek et al., Theor. Appl. Genet. 103:797-803 (2001)). Specialized disease screening sites can be costly to operate, and plants must be grown to maturity in order to classify the level of tolerance. In contrast, selection through the use of molecular markers associated with GLS tolerance has the advantage of permitting at least some selection based solely on the genetic composition of the progeny. Thus, GLS tolerance can be measured very early on in the plant life cycle, even as early as the seed stage. The increased rate of selection that can be obtained through the use of molecular markers associated with the GLS tolerance trait means that plant breeding for GLS tolerance can occur at a faster rate and that commercially acceptable GLS tolerant plants can be developed more quickly.